In the purification of oxidation products, as, for example, terephthalic acid derived from the oxidation of p-xylene, major impurities include partially oxidized compounds, such as 4-carboxybenzaldehyde. This impurity can be substantially removed by hydrogenation of the aldehyde functional group to a methylol group or by decarbonylation with removal of the carbon-containing substituent group. Such modification of the impurity components converts them to products which can be more readily removed from the desired terephthalic acid product than can the original impurity components from which they were formed.
In purification steps, as outlined above, a customary catalyst consists of palladium metal finely dispersed upon granules of activated coconut shell charcoal. Although such catalysts possess generally acceptable physical properties, the carbon granulates abrade easily, thus causing a new and different type of product contamination as well as leading to poor recovery of the palladium component.
Both palladium recovery and catalyst fines contamination are related to catalyst physical properties such as crush strength, abrasion resistance and fines content. There remains a need, and it is an object of this invention to provide, a catalyst composition with a better catalyst support having improved physical properties. Other objects and advantages of the present invention will become apparent upon reading the following detailed description and appended claims.
It has now been found that high-activity metal catalysts, having improved physical properties, may be obtained by the use of metallic support, particularly by the use of a porous, sintered metal as the catalyst substrate. Sintered metals are readily available so that such catalysts become practical and economically attractive.
In the past, metal catalysts have been modified in various ways to achieve improved performance or improved physical characteristics. For example, U.S. Pat. No. 3,123,574 discloses the treatment of a palladium/charcoal catalyst with a salt of silver, mercury, or bismuth to improve hydrogenation of fatty oils. In U.S. Pat. No. 3,928,237, catalysts for suppressing emissions from internal combustion engines are prepared by first absorbing a metal-ammonia complex on a selected substrate, preferably alumina, and then decomposing the complex to afford the free metal. Separate preparations of different metals may be combined in use. In U.S. Pat. No. 4,187,200, an alloy of two metals is created on a support surface, one of the metals being converted to a volatile compound and removed to leave an active catalytic metal in porous form on a support which may be metallic or non-metallic.